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Abstract:

The present invention provides a power converter. The power converter
includes a transformer, a power switch and a controller. The transformer
has a primary winding, a secondary winding and an auxiliary winding. The
power switch is coupled to the primary winding of the transformer to
regulate the power converter. The controller has an output terminal for
generating a driving signal to switch the power switch in response to a
switching signal. A thermal resistor is coupled to the output terminal of
the controller. The driving signal is adjusted across the thermal
resistor during an off-period of the switching signal.

Claims:

1. A controller for a power converter, comprising: a switching circuit
generating a switching signal; a driving circuit formed by a high-side
transistor and a low-side transistor to generate a driving signal,
wherein said driving signal is utilized to regulate said power converter;
an over-temperature protection circuit coupled to said driving circuit,
wherein a thermal resistor is coupled to said driving circuit, and said
driving signal is adjusted across said thermal resistor during an
off-period of said switching signal; and a signal generator coupled to
control said over-temperature protection circuit, wherein said signal
generator is further coupled to said driving circuit for driving said
high-side transistor and said low-side transistor.

2. The controller as claimed in claim 1, wherein said over-temperature
protection circuit comprises: a current-source circuit enabled to provide
a current to said thermal resistor for adjusting said driving signal
across said thermal resistor in response to a first signal generated by
said signal generator during said off-period of said switching signal;
and a comparator comparing a threshold voltage with said driving signal
during said off-period of said switching signal to generate an
over-temperature signal, wherein when said over-temperature signal is
enabled for longer than a delay time, said driving signal will be
disabled to turn off said power converter.

3. The controller as claimed in claim 2, wherein said signal generator
comprises: a saw-tooth circuit, generating a first driving signal and a
saw-tooth signal in response to said switching signal, wherein said first
driving signal is utilized to drive said high-side transistor; a first
comparison circuit, comparing said saw-tooth signal with a first
reference voltage for disabling a second driving signal, wherein said
second driving signal is enabled to drive said low-side transistor in
response to a falling-edge of said first driving signal. an enabling
circuit, generating said first signal in response to a falling-edge of
said second driving signal, wherein said first signal is utilized to
enable said over-temperature protection circuit; and a second comparison
circuit, comparing said saw-tooth signal with a second reference voltage
for enabling a second signal, wherein said second signal is utilized to
disable said over-temperature protection circuit.

4. The controller as claimed in claim 1, wherein said thermal resistor is
a negative temperature coefficient resistor, and a level of said driving
signal decreases as an environmental temperature of said power converter
increases during said off-period of said switching signal.

5. The controller as claimed in claim 1 further comprising a clamping
circuit enabled to couple to said thermal resistor for clamping a maximum
magnitude of said driving signal during said off-period of said switching
signal.

6. A power converter comprising: a transformer having a primary winding,
a secondary winding, and an auxiliary winding; a power switch coupled to
said primary winding of said transformer to regulate said power
converter; and a controller having an output terminal for generating a
driving signal to switch said power switch in response to a switching
signal, wherein a thermal resistor is coupled to said output terminal of
said controller.

7. The power converter as claimed in claim 6, wherein said driving signal
is adjusted across said thermal resistor during an off-period of said
switching signal.

8. The power converter as claimed in claim 7, wherein said controller
comprises: a switching circuit generating said switching signal; an
over-temperature protection circuit coupled to said thermal resistor for
adjusting said driving signal across said thermal resistor during said
off-period of said switching signal; and a clamping circuit enabled to
couple to said thermal resistor to clamp a maximum magnitude of said
driving signal during said off-period of said switching signal.

9. The power converter as claimed in claim 6, wherein said thermal
resistor is a negative temperature coefficient resistor.

10. A method of providing over-temperature protection for a power
converter, comprising steps of: providing a switching signal; generating
a driving signal to switch a power switch in response to said switching
signal for regulating said power converter; generating a saw-tooth signal
during an off-period of said switching signal; enabling an
over-temperature protection circuit as said saw-tooth signal exceeds a
first reference voltage; adjusting said driving signal in response to an
environmental temperature of said power converter; comparing said driving
signal with a threshold voltage to enable a delay signal for generating
an over-temperature signal after a delay time; and disabling said
over-temperature protection circuit as said saw-tooth signal exceeds a
second reference voltage.

11. The method as claimed in claim 10, wherein said driving signal is
adjusted under a maximum magnitude during said off-period of said
switching signal.

12. The method as claimed in claim 11, wherein said maximum magnitude is
used to avoid said power switch being turned on during said off-period of
said switching signal.

13. The method as claimed in claim 10, wherein said over-temperature
signal is used for turning off said power converter.

14. The method as claimed in claim 10, wherein said second reference
voltage is greater than said first reference voltage.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to PWM (Pulse Width
Modulation) controllers, and, more specifically, the present invention
relates to a PWM controller having over-temperature protection for power
converters.

[0003] 2. Description of the Related Art

[0004]FIG. 1 shows a conventional power converter utilizing a controller
60 having over-temperature protection. The power converter comprises a
transformer 10, a power transistor 20, a resistor 25, the controller 60,
a thermal resistor 36, rectifiers 11 and 21, capacitors 12 and 22, and a
secondary feedback circuit 16. The controller 60 has a supply terminal
VCC, an output terminal GATE, a sense terminal CS, a feedback terminal
FB, a temperature detection terminal RT, and a ground terminal GND. The
transformer 10 comprises a primary winding NP, an auxiliary winding
NA, and a secondary winding NS. The auxiliary winding NA
charges the capacitor 22 via the rectifier 21 to generate a supply
voltage VCC for powering the controller 60. The secondary winding
NS generates an output voltage VO of the power converter across
the capacitor 12 via the rectifier 11. The secondary feedback circuit 16
comprises a resistor 13, a zenor diode 14, and an opto-coupler 15. The
resistor 13 is connected between an output of the power converter and a
cathode of the zenor diode 14. An input of the opto-coupler 15 is
connected to an anode of the zenor diode 14. The secondary feedback
circuit 16 receives the output voltage Vo at the output of the power
converter to generate a feedback signal VFB. The resistor 25 is
connected between a source of the power transistor 20 and a ground
reference. As the power transistor 20 is turned on by the output terminal
GATE of the controller 60, the resistor 25 will convert a switching
current IP flowing through the power transistor 20 into a sense
voltage VCS.

[0005] The controller 60 comprises an over-temperature protection circuit,
an over-current protection circuit, a regulating circuit, and a PWM
(Pulse Width Modulation) circuit 30. The over-temperature protection
circuit comprises a current source 34, a comparator 33, and a delay
circuit 35. The current source 34 is connected to the temperature
detection terminal RT of the controller 60 and a negative terminal of the
comparator 33. The thermal resistor 36 is connected between the
temperature detection terminal RT of the controller 60 and the ground
reference. A positive terminal of the comparator 33 is supplied with a
threshold voltage VT. An output of the comparator 33 generates an
over-temperature signal SOT via the delay circuit 35. The
over-current protection circuit comprises a comparator 31 having a
positive terminal supplied with a limit voltage VLMT and a negative
terminal supplied with the sense voltage VCS. An output of the
comparator 31 generates an over-current signal SOC. The regulating
circuit comprises a comparator 32 and a resistor 37. A positive terminal
of the comparator 32 receives the feedback signal VFB and is pulled
high via the resistor 37. A negative terminal of the comparator 32
receives the sense voltage VCS. An output of the comparator 32
generates a regulating signal SRG. The PWM circuit 30 receives the
over-current signal SOC, the regulating signal SRG and the
over-temperature signal SOT to generate a driving signal VG at
the output terminal GATE of the controller 60.

[0006]FIG. 2 shows an example of the PWM circuit 30 of the controller 60.
The PWM circuit 30 comprises an oscillator 301, an inverter 302, a
flip-flop 303, an AND gate 304, NAND gates 305 and 306, a blanking
circuit 307, and a buffer 308. An input of the inverter 302 receives an
oscillation signal PLS generated by the oscillator 301. An output of the
inverter 302 is connected to a clock-input ck of the flip-flop 303 to
enable the flip-flop 303. The output of the inverter 302 is also
connected to a first input of the AND gate 304. An input D of the
flip-flop 303 is supplied with the over-temperature signal SOT. An
output Q of the flip-flop 303 is connected to a second input of the AND
gate 304. An output of the AND gate 304 generates a switching signal
SPWM. The over-current signal SOC and the regulating signal
SRG are supplied to inputs of the NAND gate 305. An output of the
NAND gate 305 is connected to a first input of the NAND gate 306. The
blanking circuit 307 is connected between the output of the AND gate 304
and a second input of the NAND gate 306. An output of the NAND gate 306
is coupled to a reset-input R of the flip-flop 303 to reset the flip-flop
303. The buffer 308 receives the switching signal SPWM for
generating the driving signal VG.

[0007] To achieve over-temperature protection in conventional controllers,
an independent pin is needed. As a result, there is a need to reduce the
pin count of the controller without scarifying original functions thereof
for cost-saving concerns.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention provides a controller for a power converter.
The controller comprises a switching circuit, a driving circuit, an
over-temperature protection circuit, and a signal generator. The
switching circuit generates a switching signal. The driving circuit is
formed by a high-side transistor and a low-side transistor to generate a
driving signal. The driving signal is utilized to regulate the power
converter. The over-temperature protection circuit is coupled to the
driving circuit. A thermal resistor is coupled to the driving circuit.
The driving signal is adjusted across the thermal resistor during an
off-period of the switching signal. The signal generator is coupled to
control the over-temperature protection circuit. The signal generator is
further coupled to the driving circuit for driving the high-side
transistor and the low-side transistor.

[0009] The over-temperature protection circuit comprises a current-source
circuit and a comparator. The current-source circuit is enabled to
provide a current to the thermal resistor for adjusting the driving
signal across the thermal resistor in response to a first signal
generated by the signal generator during the off-period of the switching
signal. The comparator compares a threshold voltage with the driving
signal during the off-period of the switching signal to generate an
over-temperature signal. When the over-temperature signal is enabled for
longer than a delay time, the driving signal will be disabled to turn off
the power converter.

[0010] The signal generator comprises a saw-tooth circuit, a first
comparison circuit, an enabling circuit, and a second comparison circuit.
The saw-tooth circuit generates a first driving signal and a saw-tooth
signal in response to the switching signal. The first driving signal is
utilized to drive the high-side transistor. The first comparison circuit
compares the saw-tooth signal with a first reference voltage for
disabling a second driving signal. The second driving signal is enabled
to drive the low-side transistor in response to a falling-edge of the
first driving signal. The enabling circuit generates the first signal in
response to a falling-edge of the second driving signal. The first signal
is utilized to enable the over-temperature protection circuit. The second
comparison circuit compares the saw-tooth signal with a second reference
voltage for enabling a second signal. The second signal is utilized to
disable the over-temperature protection circuit. The thermal resistor is
a negative temperature coefficient resistor. A level of the driving
signal decreases as an environmental temperature of the power converter
increases during the off-period of the switching signal.

[0011] The controller further comprises a clamping circuit. The clamping
circuit is enabled to couple to said thermal resistor for clamping a
maximum magnitude of said driving signal during said off-period of said
switching signal.

[0012] The present invention also provides a power converter which
comprises a transformer, a power switch, and a controller. The
transformer has a primary winding, a secondary winding, and an auxiliary
winding. The power switch is coupled to the primary winding of the
transformer to regulate the power converter. The controller has an output
terminal for generating a driving signal to switch the power switch in
response to a switching signal. A thermal resistor is coupled to the
output terminal of the controller. The driving signal is adjusted across
the thermal resistor during an off-period of the switching signal.

[0013] The controller comprises a switching circuit, an over-temperature
protection circuit, and a clamping circuit. The switching circuit
generates the switching signal. The over-temperature protection circuit
is coupled to the thermal resistor for adjusting the driving signal
across the thermal resistor during the off-period of the switching
signal. The clamping circuit is enabled to couple to the thermal resistor
to clamp a maximum magnitude of the driving signal during the off-period
of the switching signal. The thermal resistor is a negative temperature
coefficient resistor.

[0014] The present invention also provides a method of providing
over-temperature protection for a power converter. The method comprises
steps of: providing a switching signal; generating a driving signal to
switch a power switch in response to the switching signal for regulating
the power converter; generating a saw-tooth signal during an off-period
of the switching signal; enabling an over-temperature protection circuit
as the saw-tooth signal exceeds a first reference voltage; adjusting the
driving signal in response to an environmental temperature of the power
converter; comparing the driving signal with a threshold voltage to
enable a delay signal for generating an over-temperature signal after a
delay time; and disabling the over-temperature protection circuit as the
saw-tooth signal exceeds a second reference voltage. The driving signal
is adjusted under a maximum magnitude during the off-period of the
switching signal. The maximum magnitude is used to avoid the power switch
being turned on during the off-period of the switching signal. The
over-temperature signal is used for turning off the power converter. The
second reference voltage is greater than the first reference voltage.

[0015] It is an objective of the present invention is to provide
over-temperature protection for power converters.

[0016] It is also an objective of the present invention to save the pin
count of the controller without scarifying the original protection
function for power converters.

[0017] It is to be understood that both the foregoing general descriptions
and the following detailed descriptions are exemplary, and are intended
to provide further explanation of the invention as claimed. Still further
objects and advantages will become apparent from a consideration of the
ensuing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention can be more fully understood by reading the
subsequent detailed description and examples with references made to the
accompanying drawings, wherein:

[0020]FIG. 2 shows a PWM circuit of a controller in the conventional
power converter of FIG. 1.

[0021]FIG. 3 shows a power converter according to the present invention.

[0022]FIG. 4 shows an embodiment of a PWM circuit of a controller
according to the present invention.

[0023]FIG. 5 shows an embodiment of a signal generator of the controller
according to the present invention.

[0024]FIG. 6 shows various waveforms of the controller according to the
present invention.

[0025] FIG. 7 shows a delay circuit of the PWM circuit according to the
present invention.

[0026]FIG. 8 shows a method of providing over-temperature protection for
the power converter according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following description is of the best-contemplated mode of
carrying out the invention. This description is made for the purpose of
illustrating the general principles of the invention and should not be
taken in a limiting sense. The scope of the invention is best determined
by reference to the appended claims.

[0028]FIG. 3 shows an embodiment of a power converter according to the
present invention. The power converter comprises a transformer 10, a
power switch 20, a resistor 25, a controller 100, a thermal resistor 36,
rectifiers 11 and 21, capacitors 12 and 22, and a secondary feedback
circuit 16. The power switch 20 is embodied with a power transistor in
the embodiment. The controller 100 has a supply terminal VCC, an output
terminal GATE, a sense terminal CS, a feedback terminal FB, and a ground
terminal GND. The transformer 10 comprises a primary winding NP, an
auxiliary winding NA, and a secondary winding NS. The auxiliary
winding NA charges the capacitor 22 via the rectifier 21 to generate
a supply voltage VCC for powering the controller 100. The secondary
winding NS generates an output voltage VO of the power
converter across the capacitor 12 via the rectifier 11. The secondary
feedback circuit 16 comprises a resistor 13, a zenor diode 14, and an
opto-coupler 15. The resistor 13 is connected between an output of the
power converter and a cathode of the zenor diode 14. An input of the
opto-coupler 15 is connected to an anode of the zenor diode 14. The
secondary feedback circuit 16 receives the output voltage VO at the
output of the power converter to generate a feedback signal VFB. The
resistor 25 is connected between a source of the power transistor 20 and
a ground reference. As the power transistor 20 is turned on by the output
terminal GATE of the controller 100, the resistor 25 will convert a
switching current IP flowing through the power transistor 20 into a
sense voltage VCS.

[0029] The controller 100 comprises an over-current protection circuit, a
regulating circuit, and a PWM circuit 50. The thermal resistor 36, which
has negative temperature coefficient characteristic, is connected to the
output terminal GATE of the controller 100. When the environmental
temperature goes high, the resistance of the thermal resistor 36 will go
low, and vice versa. The over-current protection circuit comprises a
comparator 31 having a positive terminal supplied with a limit voltage
VLMT and a negative terminal supplied with the sense voltage
VCS. An output of the comparator 31 generates an over-current signal
SOC. The regulating circuit comprises a comparator 32 and a resistor
37. A positive terminal of the comparator 32 receives the feedback
voltage VFB and is pulled high via the resistor 37. A negative
terminal of the comparator 32 receives the sense voltage VCS. An
output of the comparator 32 generates a regulating signal SRG. The
over-current signal SOC and the regulating signal SRG are
supplied to the PWM circuit 50 for generating a driving signal VG.

[0030]FIG. 4 shows an embodiment of the PWM circuit 50 of the controller
100 according to the present invention. The PWM circuit 50 comprises a
switching circuit, an over-temperature protection circuit, a clamping
circuit, a signal generator 70, and a driving circuit. The switching
circuit comprises an oscillator 301, an inverter 302, a flip-flop 303, an
AND gate 304, NAND gates 305 and 306, and a blanking circuit 307. The
over-temperature protection circuit comprises a current source 311, a
switch 312, a comparator 310, a NAND gate 309, and a delay circuit 319,
wherein the current source 311 and the switch 312 form a current-source
circuit. The driving circuit comprises buffers 315 and 317 and
transistors 316 and 318. The clamping circuit, in one embodiment of the
present invention, is embodied with a switch 313 and a diode 314.

[0031] The oscillator 301 generates an oscillation signal PLS. The
oscillation signal PLS is supplied to a clock-input ck of the flip-flop
303 and a first input of the AND gate 304 via the inverter 302. An output
Q of the flip-flop 303 is connected to a second input of the AND gate
304. The over-current signal SOC and the regulating signal SRG
are supplied to inputs of the NAND gate 305. An output of the NAND gate
305 is connected to a first input of the NAND gate 306. An output of the
AND gate 304 generates a switching signal SPWM. A second input of
the NAND gate 306 receives the switching signal SPWM via the
blanking circuit 307. An output of the NAND gate 306 is coupled to a
reset-input R of the flip-flop 303 to reset the flip-flop 303.

[0032] The switching signal SPWM is supplied to the signal generator
70 for generates a first driving signal SH, a second driving signal
SL, a first signal S1, and a second signal S2. A first
terminal of the current source 311 is connected to the supply voltage
VCC. The switch 312 and the switch 313 are connected in series
between a second terminal of the current source 311 and an anode of the
diode 314. A cathode of the diode 314 is connected to the ground
reference. Both of the switches 312 and 313 are controlled by the first
signal S1. The transistor 316 and the transistor 318 are connected in
series between the supply voltage VCC and the ground reference,
wherein the transistor 316 coupled to the supply voltage VCC is
referred to as a high-side transistor, while the transistor 318 coupled
to the ground reference is referred to as a low-side transistor. The
transistor 316 is controlled by the first driving signal SH via the
buffer 315. The transistor 318 is controlled by the second driving signal
SL via the buffer 317. A joint of the transistors 316 and 318 is
connected to the output terminal GATE of the controller 100 for
generating a driving signal VG. A joint of the switches 312 and 313
is also connected to the output terminal GATE of the controller 100. The
output terminal GATE of the controller 100 is connected to a positive
terminal of the comparator 310. A negative terminal of the comparator 310
is supplied with a threshold voltage VT. An output terminal of the
comparator 310 is connected to a first input of the NAND gate 309. A
second input of the NAND gate 309 is supplied with the second signal
S2. An output of the NAND gate 309 provides a delay signal SD
to an input of the delay circuit 319.

[0033]FIG. 5 shows an embodiment of the signal generator 70 of the
controller 100 according to the present invention. The signal generator
70 comprises a saw-tooth circuit, a first comparison circuit, a second
comparison circuit, and an enabling circuit. The saw-tooth circuit
comprises inverters 711 and 712, a current source 713, switches 714 and
715, and a capacitor 716. The first comparison circuit comprises a
comparator 717 and a NOR gate 720. The second comparison circuit
comprises a comparator 718 and a NOR gate 722. The enabling circuit, in
an embodiment of the present invention, is embodied with a NOR gate 721.
A first terminal of the current source 713 is connected to the supply
voltage VCC. The switch 714 and the switch 715 are connected in
series between a second terminal of the current source 713 and the ground
reference. The switch 715 is controlled by the switching signal
SPWM. An input of the inverter 712 receives the switching signal
SPWM. An output of the inverter 712 is connected to an input of the
inverter 711.

[0034] An output of the inverter 711 generates first driving signal
SH. The switch 714 is controlled by the switching signal SPWM
via the inverter 712. The capacitor 716 is connected in parallel with the
switch 715. A ramp voltage VRMP is obtained across the capacitor
716. The ramp voltage VRMP is supplied to a positive terminal of the
comparator 717 and a negative terminal of the comparator 718. A negative
terminal of the comparator 717 and a positive terminal of the comparator
718 are respectively supplied with a first reference voltage VR2 and
a second reference voltage VR2. First inputs of the NOR gates 720,
721, and 722 are supplied with the first driving signal SH. A second
input of the NOR gate 720 is connected to an output of the comparator
717. A second input of the NOR gate 722 is connected to an output of the
comparator 718. An output of the NOR gate 720 generates the second
driving signal SL. The second driving signal SL is supplied to
a second input of the NOR gate 721. An output of the NOR gate 721 and an
output of the NOR gate 722 respectively generate the first signal S1
and the second signal S2.

[0035]FIG. 6 shows various waveforms of the controller 100 according to
the present invention. Further referring to FIG. 5, as the switching
signal SPWM is disabled, the switch 714 will be turned on, and the
switch 715 will be turned off The current source 713 will charge the
capacitor 716 via the switch 714 to generate the ramp voltage VRMP
across the capacitor 716. Referring to FIG. 6, the ramp voltage VRMP
acts a saw-tooth signal. Once the switching signal SPWM is disabled,
the first driving signal SH will be disabled. The second driving
signal SL will be enabled in response to a falling-edge of the first
driving signal SH. Also referring to FIG. 4, the transistor 318 is
turned on by the second driving signal SL via the buffer 317. The
driving signal VG will be pulled down to a level of the ground
reference (such as 0V) in response thereto. Once the ramp voltage
VRMP continues to increase and exceeds a level of the first
reference voltage VR1, the second driving signal SL will be
disabled. The first signal S1 will be enabled in response to a
falling-edge of the second driving signal SL. This will turn off the
transistor 318 and turn on the switches 312 and 313.

[0036] Referring to FIG. 3, the thermal resistor 36 is connected between
the output terminal GATE of the controller 100 and the ground reference.
As the switches 312 and 313 are turned on by the first signal S1, a
series connected parasitic resistor of the diode 314 (not shown) will be
connected in parallel with the thermal resistor 36 via the output
terminal GATE of the controller 100. Since the resistance of the
parasitic resistor of the diode 314 is relatively lower than that of the
thermal resistor 36, most current provided by the current source 311 will
flow to the diode 314. In the meantime, the level of the driving signal
VG will then be pulled up to a level of a forward voltage VF of
the diode 314. The forward voltage VF of the diode 314 ensures a
maximum magnitude of the driving signal VG to avoid the power
transistor 20 being turned on during the off-period of the switching
signal SPWM. The threshold voltage VT supplied to the negative
terminal of the comparator 310 defines an over-temperature condition, and
the threshold voltage VT is lower than the forward voltage VF
of the diode 314. Since the thermal resistor 36 has negative temperature
coefficient characteristic, a voltage level across the thermal resistor
36, which is also equal to the level of the driving signal VG, will
decrease as the environmental temperature increases. As the level of the
driving signal VG becomes lower than the forward voltage VF of
the diode 314, most current provided by the current source 311 will flow
to the thermal resistor 36. As the level of the driving signal VG
continuously decreases in response to the increase of the environmental
temperature and becomes lower than the threshold voltage VT (for
example, at the time point indicated by "A" in FIG. 6), the comparator
310 will enable the delay signal SD via the NAND gate 309. Once the
delay signal SD is enabled for longer than a delay time provided by
the delay circuit 319, the delay circuit 319 will generate a logic-low
over-temperature signal SOT to an input D of the flip-flop 303 for
disabling the switching signal SPWM, which will further disable the
driving signal VG to cut off the energy transferring and protect the
power converter from damages caused by over-temperature conditions.

[0037] Once the ramp voltage VRMP continues to increase and exceeds a
level of the second reference voltage VR2, the second signal S2
will be enabled. The enabled second signal S2 will disable the delay
signal SD via the NAND gate 309. As long as the second input of the
NAND gate 309 is supplied with a disabled second signal S2 the
over-temperature protection circuit will be disabled. As a result, the
over-temperature protection circuit is enabled by the first signal
S1 and is disabled by the second signal S2.

[0038] FIG. 7 shows the delay circuit 319 of the PWM circuit 50 according
to the present invention. The delay circuit 319 operates substantially as
a counter comprising cascaded flip-flops 321, 322, and 326. The
clock-inputs ck of these cascaded flip-flops are supplied with the
oscillation signal PLS. Reset-inputs R of these cascaded flip-flops are
supplied with the delay signal SD. As the delay signal SD is
enabled, the oscillation signal PLS will drive these flip-flops to
generate the delay time. In one embodiment of the present invention, the
switching period is 10 μs and the delay time is 10 ms. After the delay
time, an inverse output Q of the flip-flop 326 will generate a logic-low
over-temperature signal SOT. As the delay signal SD is
disabled, these flip-flops will be reset, and the over-temperature signal
SOT will become logic-high again.

[0039]FIG. 8 shows a method of providing over-temperature protection for
a power converter according to the present invention. Firstly, a
switching signal is provided (Step 1001). Next, a driving signal is
generated to switch a power switch in response to the switching signal
for regulating the power converter (Step 1002). Next, a saw-tooth signal
is generated during an off-period of the switching signal (Step 1003).
Next, an over-temperature protection circuit is enabled as the saw-tooth
signal exceeding a first reference voltage (Step 1004). Next, the driving
signal is adjusted in response to an environmental temperature of the
power converter (Step 1005). Next, the driving signal is compared with a
threshold voltage to enable a delay signal for generating an
over-temperature signal after a delay time (Step 1006). Last, the
over-temperature protection circuit is disabled as the saw-tooth signal
exceeding a second reference voltage (Step 1007). The driving signal is
adjusted under a maximum magnitude during the off-period of the switching
signal. The over-temperature signal is used for turning off the power
converter. The maximum magnitude is used to avoid the power switch being
turned on during the off-period of the switching signal. The second
reference voltage is greater than the first reference voltage.

[0040] While the invention has been described by way of example and in
terms of the preferred embodiments, it is to be understood that the
invention is not limited to the disclosed embodiments. To the contrary,
it is intended to cover various modifications and similar arrangements
(as would be apparent to those skilled in the art). Therefore, the scope
of the appended claims should be accorded the broadest interpretation so
as to encompass all such modifications and similar arrangements.